Frequency-compensated access structure for cavity resonator



D. E. GIELOW July 11, 1967 FREQUENCY-COMPENSATED ACCESS STRUCTURE FORCAVITY RESONATOR 2 Sheets-Sheet 1 Original Filed Dec. 15, 1962 IVENTORDAVID E. GIELOW ATTORNEY D. E. GIELOW July 11, 1967FREQUENCY-COMPENSATED ACCESS STRUCTURE FOR CAVITY RESONATOR 2Sheets-Sheet 2 Original Filed Dec. 13, 1962 INVENTOR. DAVID E. GIELOWATTORNEY United States Patent 3,331,017 FREQUENCY-COMPENSATED ACCESSSTRUC- TURE FOR CAVITY RESONATOR David E. Gielow, Los Altos, Calif.,assignor to Varian Associates, Palo Alto, Calif., a corporation ofCalifornia Continuation of application Ser. No. 244,406, Dec. 13, 1962.This application June 14, 1966, Ser. No.

9 Claims. (Cl. 324-.5)

This application is a continuation of application Ser. No. 244,406,filed Dec. 13, 1962, by David E. Gielow and now abandoned.

The present invention relates in general to cavity resonators and inparticular to novel apparatus for providing access to cavity resonatorsand compensating for the change of cavity resonant frequency resultingtherefrom.

A difficulty with prior-art structures for providing access to aresonant cavity is that when a wall of the cavity resonator is aperturedtherethrough, the interior microwave field surrounding the aperturepenetrates outward through the aperture so that the effective microwavelength of the cavity is increased, thereby decreasing the operativemicrowave resonant frequency of the cavity.

In many applications, it is desirable to keep the operative microwaveresonant frequency fixed regardless of whether the cavity wall structureis solid or apertured. Apertured wall structures are needed whichconveniently may be substituted for solid wall structures withoutalteration of the cavity resonance frequency.

One object of the present invention, therefore, is provision offrequency-shift-compensated adapter means for permitting access to acavity resonator without altering its microwave resonance frequency.

One advantage of the present invention is its usefulness in connectionwith cavity resonators suitable for electron paramagnetic resonance(EPR) studies. For experiments in which paramagnetic samples are exposedto the simultaneous influence of light irradiation and a microwaveresonance field within a cavity resonator, unobstructed open accessmeans are needed so that irradiation over the entire active portion ofthese samples may be achieved. Prior-art slotted or partitioned accessmeans permit irradiation over only about 50 percent of the activeportion of a sample.

One feature of the present invention, therefore, is provision ofwaveguide access means in combination with frequency-compensated adaptormeans which may be attached to a resonant cavity for permitting externalirradiation over substantially all of the active portion of a sampledisposed within the cavity resonator without altering the microwaveresonant frequency at which the cavity operates.

A further advantage of the present invention is that it convenientlypermits light to be transmitted through a cavity resonator and permitsoptical transmission changes in a paramagnetic sample disposed withinthe cavity to be detected so that a cavity resonator may be adapted foroperation at the same fixed frequency in optical transmission resonancestudies as in other microwave resonance experiments.

One feature of the present invention, therefore, is provision ofWaveguide access means and apertured adaptor means which, in combinationwith a half-wavelength cavity coupling section on top of which amicrowave coupler and a Waveguide assembly are mounted, comprise anaccessory apparatus suitable for opening a cavity resonator havingclosed end walls without altering its resonant frequency in order thatlight may be transmitted therethrough.

These and other features and advantages of the present invention will bemore apparent after a perusal of the 3,331,017 Patented July 11, 1967ice following specification taken in connection with the accompanyingdrawings wherein the same numeral is used in the various figures for thesame or analogous element, and:

FIG. 1 is an isometric view of a frequency-shift-compensated adaptorassembly having rectangular waveguide access means in accordance withthe present invention.

FIG. 1A is an isometric view, partially broken away, of a cavityresonator, at one end of which the adaptor assembly shown in FIG. 1 isattached.

FIG. 2 is an isometric view partially broken away, of afrequency-shift-compensated adaptor assembly having a cylindricalwaveguide access means in accordance with the present invention.

FIG. 3 is an isometric view, partially broken away, of an adaptorassembly and cavity resonator combination, adapted to be disposedbetween the pole pieces of external magnet means and suitable forpermitting microwave resonant samples contained in a sample tube holderdisposed within the resonator, to be exposed to the simultaneousinfluence of light irradiation and a microwave resonance field, inaccordance with the present invention.

FIG. 4 is an isometric view of a solid rectangularly shaped end-closingstructure which is interchangeable with the adaptor assembly of FIG. 3.

FIG. 5 is an isometric view, partially broken away, offrequency-shift-compensated adaptor assemblies in combination with arectangular cavity resonator, which combination is adapted to bedisposed between the pole pieces of external magnet means and issuitable for transmitting light at one or more frequencies throughmicrowave resonant samples disposed within the cavity or for detectingoptical transmission changes within said samples while simultaneouslyobserving electron resonances.

Referring now to FIG. 1, frequency-shift-compensated adaptor assembly 1consists of rectangular waveguide access means 2 having an interiorhollow chamber which has a cut-off frequency beyond the frequency of thecavity resonator, and adaptor 3. Adaptor assembly 1 may be connected toa cavity resonator 4 as shown in FIG. 1A. In this embodiment, arectangular aperture extends through the central portion of adaptor 3,this aperture having dimensions 5 and 6 equal to the interior dimensions7 and 8, respectively, of waveguide access means 2. On the side ofadaptor 3 disposed exterior to the cavity, this aperture widens, havingdimensions equal to the outer dimensions of waveguide access means 2, sothat access means 2 may be inserted into adaptor 3 and soldered orbrazed thereto.

Waveguide access means 2, the hollow interior of which opens into theinterior of cavity resonator 4, will not propagate microwaves at themicrowave resonance frequency of cavity resonator 4. Thus cavityresonator 4 is opened Without any appreciable attenuation of theinterior microwave field, so that, for example, microwave resonantsamples disposed within the cavity may be simultaneously exposed toexternal irradiation and microwave resonance fields therein.

Adaptor 3 provides for frequency compensation throughout the operativefrequency range of cavity resonator 4 so that it remains resonant at thefrequency at which it is intended to be operated. Frequency compensationis achieved by virtue of the effect upon the cavity microwave fields ofmetallic member 9, a rectangular liplike portion of adaptor 3 whichprojects axially into cavity resonator 4 as shown in FIG. 1A. Metallicmember 9 counteracts the fringing effects of the interior microwavefield in the vicinity of aperture 10, reducing the effective microwavelength of cavity resonator 4 by an amount 0 equal to the increase of theelfective microwave length due to opening the resonator to the exterior.

Referring now to FIG. 2, another embodiment of the invention is shown inwhich waveguide access means 2' has cylindrical symmetry instead ofrectangular symmetry. Adaptor 3' is cylindrically apertured through thecentral portion of rectangular lip-like metallic member 11, which isadapted to slidably fit into rectangular cavity resonator 4. Thediameter of aperture 12 is equal to the interior diameter of accessmeans 2. The use of this combination is advantageous in connection withapplications requiring a very short waveguide section. Where, however,maximal access cross sectional area is sought as, for example, inexperiments in which it is desired to shine light through the hollowaccess chamber into a cavity resonator, the embodiment shown in FIG. 1is preferred because the rectangular symmetry provides the maximumaccess cross sectional area consistent with the requirement that thehollow access chamber have a cut-off frequency beyond the frequency ofthe cavity resonator.

Referring now to FIG. 3, a cavity resonator assembly 14 is shownconsisting of a rectangular cavity resonator 15, afrequency-shift-compensated adaptor assembly 1 comprising a rectangularadaptor 3 and rectangular waveguide access means 2, a cavity couplingsection 16, and a rectangular waveguide assembly 17 for transmittingenergy to and from resonator 15.

A hollow sample cell 18 may be introduced centrally into cavity throughsample access means 19. In one embodiment, the sample celladvantageously comprises a quartz tube having a pancake-shaped centralportion lying in a nodal plane of electric field, a position of maximummicrowave magnetic field in cavity resonator 15. The sample tube maycontain a microwave resonant sample in solid form or in solution, thesample being simultaneously exposed to the influence of the cavitymicrowave magnetic field and external irradiation without appreciableattenuation of the interior microwave field. The solution, for example,may flow through the sample tube thereby permitting process controlmeasurements determinative of the nature and amount of various materialsin the sample solution. Moreover, solutions containing different samplesmay be mixed within a tubular mixing chamber (not shown) which may beinserted into the sample cell, whereby steady state intermediates may beobserved and reaction rates determined. Alternatively, electrodes (notshown) may be disposed within the sample tube and an electrolyticreaction may be produced in a sample solution, which is simultaneouslyexposed to the influence of a cavity microwave magnetic field andexternal irradiation, electron resonances of reaction products beingobserved and reaction rates being measured.

A static D.C. magnetic field generated between the pole pieces 20 ofexternal magnet means is superimposed on cavity resonator 15substantially at right angles to the microwave magnetic field therein,coils 20' being provided on the cavity wall to impose an A.-C.modulation on the static D.C. field in order to use A.-C. detectionmeans as is well known in the art. A beam of light emanating from lightsource 21 is directed through hollow waveguide access means 2 intoresonator 15 so that substantially all of the active portion of thesample contained within sample cell. 13 is irradiated, whereby theeffect of irradiation upon electron resonances is substantiallyenhanced.

It is desirable, moreover, that cavity resonator 15 be suitable for usein connection with other resonance experiments as well as thoseinvolving light irradiation. For this reason adaptor assembly 1 isdetaachable and may be replaced, for example, by a solid end-closingend-plate 13. The substitution of such an end-plate for adaptor 1 may beaccomplished expeditiously without altering the resonant frequency ofthe cavity. Since the frequency shift associated with fringing fieldeffects occurring whenever a closed cavity is opened is eliminatedwithin adaptor assembly 1, assembly 1 may be detached from cavityresonator 15, being replaced by end-closing metallic structure 13, andsimilarly may be reattached to the cavity without detuning the cavity.

Referring now to FIG. 5, a cavity resonator assembly 22 is shown inwhich two frequency-shift-compensated adaptors are used. The assemblyshown in FIG. 3 conveniently may be adapted to permit light transmissionexperiments by the addition of a frequency-shift-compensated adaptor anda half wavelength coupling section on top of which a waveguidetransmission line is mounted. Half wavelength cavity coupling section 23is attached to one end of cavity resonator 15, waveguide assembly 17 andcavity coupling section 16 having been removed. A microwave coupler 24is mounted on top of this half wavelength section for coupling energybetween waveguide transmission line 25 and cavity resonator 15. Acoupler which is especially suitable for this purpose is the coupler ofJohn C. Everitt which is the subject of a copending patent application,Ser. No. 228,113, filed Oct. 3, 1962, now Patent No. 3,214,684 andassigned to applicants assignee. An additionalfrequency-shift-compensated adaptor assembly 26 is connected to halfwavelength section 23 thereby permitting light from light source 21 tobe transmitted through a sample disposed centrally within cavityresonator 15. A detector 27 is adapted to receive light transmittedthrough the resonator, whereby measurements of optical absorption may bemade while simultaneously observing the electron resonances of thesample. Alternatively, detector 27 may be replaced by another lightsource so that, for example, light at two different frequencies may betransmitted through the interior of the resonator while electronresonances are observed.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A microwave apparatus comprising, in combination:

(a) a microwave cavity assembly comprising a cavity resonator, at leastthree walls of which are apertured therethrough, and sample access meansforming a hollow interior chamber having a cut-off frequency beyond thefrequency of said resonator, said sample access means extending throughtwo oppositely-disposed apertured walls of said resonator and beingadapted to support a sample cell within said resonator, and said samplecell being adapted to contain a microwave resonant sample therein,

(b) at least one frequency-shift-compensated adaptor assemblycomprising:

(1) a short hollow waveguide section forming a hollow chamber having acut-off frequency beyond the resonant frequency of said cavityresonator, for providing access to the interior of said resonatorthrough a wall adjacent said oppositely disposed walls while producingtotal attenuation of interior microwave fields extending from saidresonator into said waveguide section, and

(2) an end-closing adaptor means comprising a solid member having acentral portion projecting axially into said resonator and aperturedtherethrough, the projecting portion of said member being productive ofa decrease in the microwave length of said cavity and a correspondingincrease in the resonant frequency of said cavity, said decrease inmicrowave length and said increase in frequency being equal and oppositerespectively to the increase in said microwave length and thecorresponding decrease in said frequency due to fringing effects of saidinterior microwave fields about said aperture,

(0) a cavity coupling section being attached at one end of said cavityresonator opposite said adaptor assembly,

(d) a Waveguide assembly adapted for transmission of microwave energycoupled to said resonator,

(e) at least one light source disposed in coaxial alignment with theinterior longitudinal axis of said short waveguide section and adaptedfor energization by external power supply means, for transmitting lightthrough said waveguide section into said resonator whereby saidmicrowave resonant sample may be irradiated therein.

2. A microwave apparatus according to claim 1 wherein said cavityresonator, said cavity coupling section, and said waveguide assembly arerectangularly shaped, said waveguide assembly being attached to saidcavity coupling section adjacent one endof said cavity coupling section.

3. A microwave apparatus according to claim 2 wherein said projectingportion of said solid member is rectangularly shaped.

4. A microwave apparatus according to claim 3 wherein said hollowwaveguide section is rectangularly shaped and said solid member isrectangularly apertured therethrough.

5. A microwave apparatus according to claim 3 wherein said hollowwaveguide section is cylindrically shaped and said solid member iscylindrically apertured therethrough.

6. A microwave apparatus according to claim 1, wherein said cavitycoupling section comprises a rectangular half-wavelength cavity section,a microwave coupler being mounted upon the top thereof, said couplerbeing adapted to couple microwave energy between said resonator and saidwaveguide assembly, said assembly and said resonator being rectangularlyshaped, and said waveguide assembly being disposed on top of saidcoupling section.

7. A microwave apparatus according to claim 6 and further including alight detector adapted for energization by external power supply meansand disposed in coaxial alignment with said light source, for measuringthe optical absorption occurring in said microwave resonant sample whenlight is transmitted therethrough.

8. A microwave apparatus for use in gyromagnetic resonance studiescomprising:

a cavity resonator having a detachable adaptor wall with an unobstructedaperture therein;

waveguide means coupled to said adaptor wall so that unrestrictedoptical radiation may be directed through said waveguide and saidaperture to the interior of said cavity resonator;

said adaptor wall having a rectangular lip-like portion surrounding saidaperture and projecting into such cavity resonator for reducing theefiective microwave length of the cavity resonator by an amountsubstantially equal to the increase of effective microwave lengthresulting from said aperture whereby the resonant frequency of saidcavity resonator is not varied by said adaptor wall.

9. A microwave apparatus comprising, in combination, a cavity resonatorand at least one adaptor means comprising a short hollow waveguidesection and a metallic field-displacing rectangular lip-like memberprojecting axially into said resonator for connecting the interior ofsaid waveguide section with the interior of said resonator through atleast one wall aperture in said resonator, whereby access is provided tothe interior of said cavity; a cavity coupling section adjacent to oneend of said resonator, and a waveguide assembly connected to saidcoupling section for transmitting microwave energy to and from saidresonator; at least one light source disposed coaxially with theinterior longitudinal axis of said hollow waveguide section fortransmitting light through said waveguide section into the resonator;and a sample tube that is centrally contracted to form a pancake-shapedcapillary disposed in the nodal plane of the electric field of saidresonator, said capillary being adapted to contain a paramagnetic samplein aqueous solution therein, the face of said pancake capillary beingdisposed in the path of the transmitted light.

No references cited.

WALTER L. CARLSON, Primary Examiner.

M. J. LYNCH, Assistant Examiner.

1. A MICROWAVE APPARATUS COMPRISING, IN COMBINATION: (A) A MICROWAVECAVITY ASSEMBLY COMPRISING A CAVITY RESONATOR, AT LEAST THREE WALLS OFWHICH ARE APERTURED THERETHROUGH, AND SAMPLE ACCESS MEANS FORMING AHOLLOW INTERIOR CHAMBER HAVING A CUT-OFF FREQUENCY BEYOND THE FREQUENCYOF SAID RESONATOR, SAID SAMPLE ACCESS MEANS EXTENDING THROUGH TWOOPPOSITELY-DISPOSES APERTURED WALLS OF SAID RESONATOR AND BEING ADAPTEDTO SUPPORT A SAMPLE CELL WITHIN SAID RESONATOR, AND SAID SAMPLE CELLBEING ADAPTED TO CONTAIN A MICROWAE RESONANT SAMPLE THEREIN, (B) ATLEAST ONE FREQUENCY-SHIFT-COMPENSATED ADAPTOR ASSEMBLY COMPRISING: (1) ASHORT HOLLOW WAVEGUIDE SECTION FORMING A HOLLOW CHAMBER HAVING A CUT-OFFFREQUENCY BEYOND THE RESONANT FREQUENCY OF SAID CAVITY RESONATOR, FORPROVIDING ACCESS TO THE INTERIOR OF SAID RESONATOR THROUGH A WALLADJACENT SAID OPPOSITELY DISPOSED WALLS WHILE PRODUCING TOTALATTENUATION OF INTERIOR MICROWAVE FIELDS EXTENDING FROM SAID RESONATORINTO SAID WAVEGUIDE SECTION, AND (2) AN END-CLOSING ADAPTOR MEANSCOMPRISING A SOLID MEMBER HAVING A CENTRAL PORTION PROJECTING AXIALLYINTO SAID RESONATOR AND APERTURED THERETHROUGH, THE PROJECTING PORTIONOF SAID MEMBER BEING PRODUCTIVE OF A DECREASE IN THE MICROWAVE LENGTH OFSAID CAVITY AND A CORRESPONDING INCREASE IN THE RESONANT FREQUENCY OFSAID CAVITY, SAID DECREASE IN MICROWAVE LENGTH AND SAID INCREASE INFREQUENCY BEING EQUAL AND OPPOSITE RESPECTIVELY TO THE INCREASE IN SAIDMICROWAVE LENGTH AND THE CORRESPONDING DECREASE IN SAID FREQUENCY DUE TOFRINGING EFFECTS OF SAID INTERIOR MICROWAVE ABOUT SAID APERTURE, (C) ACAVITY COUPLING SECTION BEING ATTACHED AT ONE END OF SAID CAVITYRESONATOR OPPOSITE SAID ADAPTOR ASSEMBLY, (D) A WAVEGUIDE ASSEMBLYADAPTED FOR TRANSMISSION OF MICOWAVE ENERGY COUPLED TO SAID RESONATOR,(E) AT LEAST ONE LIGHT SOURCE DISPOSED IN COAXIAL ALIGNMENT WITH THEINTERIOR LONGITUDINAL AXIS OF SAID SHORT WAVEGUIDE SECTION AND ADAPTEDFOR ENERGIZATION BY EXTERNAL POWER SUPPLY MEANS, FOR TRANSMITTING LIGHTTHROUGH SAID WAVEGUIDE SECTION INTO SAID RESONATOR WHEREBY SAIDMICROWAVE RESONANT SAMPLE MAY BE IRRADIATED THEREIN.